There do not appear to be any systems in the prior art which provide for error correction for the contents of random access magnetic memories which have been subjected to a nuclear radiation event, and other phenomena that produces an error in a word of memory. Random access memories which must retain vital data, even after being subjected to nuclear radiation while writing or restoring data, can be rendered immune to the radiation effects, by storing small quantities of data doubly or triply redundantly in the memory. This enables a single word loss to be remedied by reference to other error-free copies of the affected word. However, practical limitations on memory sizes prohibits the entire contents of a typical memory from being stored in a double or triple redundant manner.
Data loss may be obviated in a random access memory without the need for redundant storage, in the case of the fixed data, by the use of the plated wire type non-destructive read-out memory. If suitable circumvent circuitry is provided to protect the unaddressed words stored in the plated wire memory, the memory will be safe from the effects of nuclear radiation insofar as the fixed data is concerned. This is because of the non-destructive read-out characteristics of this type of memory. The plated wire memory, therefore, may be used for the storage of fixed data, since under normal operation, the fixed data will be read only from the memory and, due to the non-destructive read-out properties of the memory, any word being read during a nuclear event can be reconstituted from the memory itself. However, the plated wire memory does present problems when used for variable data storage, since it is vulnerable insofar as a word being written into memory is concerned.
Thus, the plated wire memory is not immune to the effects of nuclear radiation which occurs when a particular variable data word is being up-dated, since such a particular data word being written into the plated wire memory during a nuclear event could be lost. Therefore, without further protection, the plated wire memory is not suitable for the storage of variable data.
Moreover, the non-destructive read-out plated wire memory is more costly than the destructive read-out magnetic core type, and significant cost problems arise when the non-destructive read-out type of memory is used for the fixed data storage. The option of using the less expensive destructive read-out memory for fixed data storage presents a problem, however, since it is susceptible to the effects of nuclear radiation on both the words being written into the memory or read from the memory. This is because the destructive read-out characteristics of the magnetic core memory creates the need for restoring data after each readout operation. This makes this type of memory vulnerable to the radiation effects when data is being read from the memory or written into the memory. Redundant storage of all the contents of a destructive read-out magnetic core memory is equally impractical.
The present invention provides various embodiments of an improved system for rendering the destructive read-out magnetic core random access memory immune to the effects of nuclear radiation, and the like, insofar as the loss of fixed or variable data words is concerned. The various embodiments to be described are applicable to the fixed data portion of the memories in which certain invariable, fixed, program data words are stored, and which remain unchanged throughout the entire computer program; and certain embodiments are also applicable to the variable data, or scratch pad, portions of such memories in which variable data words are stored that are subject to up-dating, or other changes, from time to time.
The system of the invention overcomes the most difficult problem of error correction due to nuclear radiation, or other interfering radiation, in the less expensive destructive read-out core memory, and of achieving this result in a relatively simple, economical and straightforward manner.
The basic problem of rendering a random access magnetic memory immune to the effects of nuclear radiation is similar for both the core memory and the plated wire memory. The primary similarity lies in the fact that during a write or restore cycle, it is extremely difficult to control the currents in either memory to the precision required to guarantee a correct write or restore operation. The primary difference between the core and plated wire memories lies in the fact that during the read cycle of a core memory, a read/restore operation is required so that the core member is also susceptible to the effects of nuclear radiation during the read operation, where the plated wire memory is not.
The system of the invention will be discussed in conjunction with a random access core in which each memory element is selected by an X-Y switching matrix, and by an applied inhibit current (I). Such a memory is well known to the art and is described, for example, at page 185, of Volume 4, McGraw Hill Encyclopedia of Science and Technology (1960 Edition).
The protective system of the invention includes circumvent circuitry which responds to the detection of nuclear radiation event to isolate magnetic core random access memory from the effects thereof. The circumvent circuitry enables unaddressed memory locations to be protected. However, it is extremely difficult by circumvent circuitry to control the required currents in the memory should exposure to the nuclear radiation occur during an actual write or read/restore operation. This means that the word being read from the core memory, or written into the memory, during the exposure may be lost. As described, the plated wire memory operates on a non-destructive read-out basis which safeguards a stored word from being destroyed during a read cycle, even if it is being read when a nuclear event occurs. In the case of the magnetic core memory, however, the word desired in each read operation must be restored in a subsequent write operation, so that the affected word may be lost during the presence of nuclear radiation. Thus, additional means must be provided to reconstruct words being accessed during the nuclear event. In the case of the plated wire memory, the additional means is required only to reconstruct a word actually being written into the memory during the nuclear event. In the case of the core memory, however, the corrective measures must be taken with respect to words being read or written during the nuclear event.
The system of the invention, as explained briefly above, in one of its embodiments provides an error correction word for each block of the fixed data words, and this error correction word serves to permit the destructive read-out core memory to meet all the performance and operational requirements of a radiation immune system with respect to the fixed data program storage. A second embodiment of the invention applies the error correction word concept to the variable data storage situation; and a third embodiment applies another error correction technique to the variable data storage.